Water loss in excess of salt deficit

(each litre dehydration = ↑5mEq Na +)

  • Patients can't drink or xs loss e.g. sweat/heat stroke
  • GI loss without replacement (vomit, diarrhoea etc.)
  • Renal disease (can't concentrate urine)
  • Diabetes insipidus
  • Osmotic diuresis (↑Glu, DKA, HONC,HypoKalaemia)

Xs salt (either ingestion or iatrogenic IV fluids)


  • Dehydration++
  • Often asymptomatic if gradual onset
  • Later weakness, ataxia, tremor, rigidity and hyperreflexia
  • May have confusion/ lethargy / coma (brain dehydration)
  • Beware - overall may have HYPOnatraemia in total body tissues


Calculate Water deficit

  • Total body water x (serum Na-140)/ (140)
  • TBW is 50% in males and 40% in females
    So male with Na of 160 has free water deficit of (0.5 x 70) x 20/140 = 5 Litres
  • Aim to replace deficit plus maintenance and ongoing losses
  • Investigate for underlying cause


  • Stop ongoing losses
  • Correct water deficit (SLOWLY [days] - avoid cerebral oedema)
  • Oral water or 5% dextrose IV (beware too rapid correction)
  • Aim for 1mmol Na+ drop / hr
  • Beware - correct Na+ deficit if dehydrated
  • Investigate for and treat underlying cause

Content by Dr Íomhar O' Sullivan 22/09/2012. Last review Dr ÍOS 15/06/21.